1. Field of the Invention
The present invention relates to an improved process for preparing a bis(3-nitrophenoxy) compound represented by the formula (II): ##STR2## wherein X is a direct bond, divalent hydrocarbon having from 1 to 10 carbon atoms or a divalent group selected from --C(CF.sub.3).sub.2 --, --CO--, --S--, --SO--, --SO.sub.2 -- and --O--.
The bis(3-nitrophenoxy) compund is an important compound used as a material for preparing a monomer which is in turn used in the preparation of heat resistant high molecular weight polymers, polyamides and polyimides in particular. For example, ether diamine obtained by reducing the compound can be used as a diamine component of polyimide which can provide adhesive having excellent heat resistance.
2. Description of the Related Art
A process for the condensation of 4,4'-bisphenols represented by the formula (I) : ##STR3## wherein X is a direct bond, divalent hydrocarbon having from 1 to 10 carbon atoms or a divalent group selected from --C(CF.sub.3).sub.2 --, --CO--, --S--, --SO--, --SO.sub.2 -- and --O--, with m-dinitrobenzene to prepare a bis(3-nitrophenoxy) compound represented by the formula (II): ##STR4## wherein X is the same as in the formula (I), has already been known in Japanese Laid-Open Patent SHO 61-194050(1986) and 62-45563(1987).
According to Japanese Laid-Open Patent SHO 61-194050, the condensation reaction can be carried out in an aprotic polar solvent in the presence of a base.
According to Japanese Laid-Open Patent SHO 62-45563, the condensation reaction can be further accelerated by using a phase transfer catalyst under the same conditions.
These processes can provide the desired bis(3-nitrophenoxy) compound without using an expensive reaction accelerator such as crown ethers. These processes, however, have the following problems.
(1) Alkali metal carbonate on alkali metal hydrogen carbonate used as a base is required in an excessive amount such as 2.0 to 2.6 moles per mole of 4,4'-bisphenol and hence is disadvantageous in industry.
(2) When the reaction is carried out according to these processes, reproducibility of the reaction is poor and consistent results cannot be obtained even though the reaction is conducted under the same conditions. Consequently, the products cannot maintain consistent quality.
(3) The reaction cannot be completed unless m-dinitrobenzene is used in an amount of from 2.3 to 2.6 moles per mole of the bisphenol. When the reaction is conducted by using a smaller amount, bisphenol is reacted with only one mole of m dinitrobenzene and the resultant mononitrophenoxy compound remains in a large amount after finishing the reaction which causes a reduction in the yield.
(4) Excess m-dinitrobenzene causes decomposition by itself or partly leads to decomposition after reacting with the bisphenol to form tarry materials or 3,3'-dinitroazobenzene, or partly remains unreacted. The desired bis(3-nitrophenoxy) compound can be separated from these by-products with relative ease and is obtained in a good purity. However, in the distillation for recovering the solvent containing these impurities unreacted m-dinitrobenzene and tarry materials derived from dinitrobenzene are concentrated at the bottom of the distillation vessel and hence lead to a danger of explosion. Consequently, efficient recycle of the reaction solvent cannot be performed, which is disadvantageous in industry.